1. Field of the Invention
The present invention relates to a technique for manufacturing a semiconductor substrate having an SOI (Silicon On Insulator) structure, and more particularly relates to a semiconductor substrate suitable for a usage in an electro-optical apparatus, and a method of manufacturing the same.
2. Description of the Related Art
The SOI technique using a silicon layer made on an insulator layer for a formation of a semiconductor device has been developed in order to make the integration degree of the semiconductor device higher, since it has excellent properties that cannot be achieved by a typical silicon substrate, such as an alpha-ray resistance, a latch-up property, a depression effect of a short channel or the like.
Recently, an excellent short channel depression effect is found out from a sample in which a device is formed on an SOI layer whose film thickness is reduced to 100 nm or less. Also, the thus-formed SOI device can have a high reliability resulting from an excellent radiation resistance and attain a high speed of an element resulting from a decrease in parasitic capacitance and a low consumption power, or make a hyperfine process rule resulting from a formation of a perfectly depleted electric field effect transistor.
As one of such SOI techniques, there is a technique of manufacturing an SOI substrate in which a single crystal silicon substrate is laminated. In this technique typically referred to as a laminating method, the single crystal silicon substrate and a support substrate are laminated to each other by using hydrogen bonding force. Then, the lamination strength is increased by a thermal process, and the film thickness of the single crystal silicon substrate is reduced by grinding, polishing, or etching. Accordingly, the single crystal silicon layer is formed on the support substrate. In this technique, the film thickness of the single crystal silicon substrate is directly reduced. Thus, it is possible to manufacture the device having the excellent crystal property of the silicon thin film and the high performance.
Also, as the application techniques of this lamination method, there are a method of implanting hydrogen ions into a single crystal silicon substrate, and laminating it onto a support substrate, and then separating a thin film silicon layer from the hydrogen implantation region of the single crystal silicon substrate by a thermal process (U.S. Pat. No. 5,374,564), and a method of epitaxially growing a single crystal silicon layer on a silicon substrate whose surface is made porous, and laminating it onto a support substrate, and then removing the silicon substrate, and etching the porous silicon layer, and accordingly forming the epitaxial single crystal silicon thin film on the support substrate (Japanese Patent Application Laid Open (JP-A-Heisei, 4-346418)) and the like.
The SOI substrate according to such a lamination method is used to manufacture various devices, similarly to a typical bulk semiconductor substrate (i.e., a semiconductor integrated circuit). However, as the difference from the conventional bulk substrate, it can be cited that various materials can be used for the support substrate. That is, as the support substrate, it is possible to use not only the typical silicon substrate but also a transparent quartz, a glass substrate or the like. As a result, for example, by forming a single crystal silicon thin film on a transparent substrate, it is possible to form an high performance transistor element by using a single crystal silicon with an excellent crystal performance, even in an electro-optical apparatus, such as a device requiring an optical transparency, for example, a transparent type of a liquid crystal display device and the like.
By the way, the usage of the above-mentioned SOI substrate in a liquid crystal apparatus enables the device formation process to be applied to the single crystal semiconductor substrate. That is, since a TFT (Thin Film Transistor) for driving a pixel electrode and a drive circuit around a display are formed on an SOI layer that is the single crystal semiconductor layer, it is possible to provide the display having a hyperfine structure and a high speed.
In the SOI substrate used in such a liquid crystal apparatus, a single crystal semiconductor layer in a region in which a pixel is formed is desired to be extremely thin in order to suppress a light leak current. On the other hand, in a region in which a drive circuit around the pixel formation region is formed, it is necessary to form a drive circuit to be driven at a high speed. Thus, it is desired that a sheet resistance is small. Hence, it is advantageous to make the single crystal semiconductor layer thicker.
However, the manufacturing method disclosed in the above-mentioned gazette can manufacture only the semiconductor substrate having a single crystal semiconductor layer of a certain thickness. For example, trying to form a single crystal semiconductor layer at a thickness of 100 nm or less that is required of a pixel formation region makes the formation of a peripheral drive circuit very difficult. On the contrary, if the whole portion is formed at a thickness of about 200 nm in order to easily form a drive circuit, it is necessary to again reduce the film thickness of the single crystal semiconductor layer in the pixel formation region. Thus, this reduction in the film thickness needs an extremely high control technique.